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From: mbeck@ai.mit.edu (Mark Becker)
Newsgroups: sci.electronics
Subject: Need help damping audio from near-ultrasonic DC-DC converter
Keywords: DC converter
Message-ID: <10355@life.ai.mit.edu>
Date: 27 Aug 90 02:12:00 GMT
Sender: news@ai.mit.edu
Lines: 33

Hello *

I have a disk drive with a very loud DC-DC converter.  The converter
is based on a TL-497 switching regulator and generates -12V from +12V.

This circuit uses an inductor in a 'flyback' configuration to generate
-12V.  The inductor whines at about 13 KHz under load and it's loud.
I've traced the circuit and, along with some judicious 'scope probing,
determined the inductance to be 100 microhenries.  It's wound on a
small audio transformer core.  Peak current through the inductor is
about 3 Amps.  (Nice of them: .2 ohm resistor in series w/inductor).

I've tried two things to eliminate/reduce the oscillator whine:

 * Decreased the oscillator capacitor in the TL-497 circuit to
   increase the frequency past, say, 18 KHz.  The output waveform
   degraded immediately (had a nasty spike in it) and the noise had a
   Bad Feel to it.

 * Removed the offending inductor, dunked it in silicone rubber, and
   re-installed it on short wires (mechanical isolation from the
   printed-circuit board).  That reduced the noise some.. but it's
   still very loud.

I would appreciate hearing from others who have solved this kind of
electro-acoustic problem.  I'd use an external -12V supply.. but do
not know the supply sequencing used in this drive.

Mark
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| Mark Becker					| .signature under	|
| Internet: mbeck@ai.mit.edu			|	construction	|
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